Since natural selection favors traits that increase fitness, it seems that populations should eventually become genetically homogeneous. But evolution isn't so one-dimensional: When researchers adjusted the color frequencies of wild guppy populations in Trinidad, they found that unusual variants — regardless of color — had higher survival rates. This is called frequency-dependent survival: selection favoring the rare and disfavoring the common, preventing a long-term homogeneity that — no matter how beneficial in the short term — might someday prove disastrous.

Nature's collection is in honor of the upcoming 200th anniversary of Darwin's birth.